The present invention relates to a microlens array sheet having a plurality of semispheroid microlenses cut from a spheroid material or semispherical microlenses cut from a spherical material, and a method of producing such a microlens array sheet. Particularly, this invention relates to a microlens array sheet useful for screens, for example, in rear-projection televisions, and a method of producing such a microlens array sheet.
Displays are classified into a direct-view type for direct-viewing images displayed on a cell assembly and a projection type for viewing images projected onto a screen by front or rear projection. The latter can be manufactured in a large type at low cost and are gradually popular, especially, in North America and Chaina.
A rear-projection display is a projection type. This type of display mostly employs a lenticular lens array sheet for its screen. The lenticular lens array sheet is, usually, produced by extrusion or injection molding, or press roll with a photocurable resin.
However, the lenticular lens array sheet has a problem in that it gives a wider angle of field only either a horizontal or vertical direction.
In order to solve such a problem, Japanese Un-examined Patent Publication No. 2001-305315 proposes a screen equipped with a light-shielding layer having a microlens array sheet with a lens function.
The microlens array sheet has microlens aligned on a flat base, as concave and convex sections of the base. A screen equipped with such a microlens array sheet is applicable to displays and expected to be popular in the near future.
Japanese Un-examined Patent Publication No. 2001-305315 discloses a microlens array sheet. Formed on a surface of a lens substrate is a plurality of microlens. Formed on the other surface of the substrate is a light-shielding layer having a circular or square light-emitting section.
The Un-examined Patent Publication gives one requirement (Sr≧2t×tanΘ+R) to the microlens array sheet, in which “Sr” is the size of the microlens, “t” is the thickness of the lens substrate, “Θ”, eqaul to sin−1 (1/n), is the maximum incident angle, “n” is a refractive index of the lens substrate, and “R” is the diameter of the light-emitting section.
However, a microlens array sheet designed according to the expression shown above sometimes causes optical vignetting, a phenomenon in which transmissivity (the amount of an outgoing beam/the amount of an incoming beam) is decreased when a part of an incoming beam incident along a correct optical path is blocked due to spherical aberration. This is because the above expression does not include a focal point of each microlens.
According to the expression, the maximum incident angle “Θ” depends the refractive index “n” of the lens substrate. Thus, an outgoing beam emitted from the light-emitting section originated from an incoming beam having the maximum incident angle “Θ” sometimes causes optical vignetting to other incoming beams.
Wider apertures for the light-shielding layer to avoid optical vignetting decreases a ratio of the area of black on the light-shielding layer to the total area of a light diffusion plate, thus causing lower contrast for projected images.
There is thus a demand for a microlens array sheet and a production method thereof that achieve both restriction of optical vignetting and enhancement of contrast inconsistent with each other.